Hepatocellular carcinoma, the major manifestation of liver cancer is a major health concern. Currently, this disease is the 3rd leading cause of cancer deaths worldwide with over 500,000 deaths each year. The high incidence of hepatocellular carcinoma development is attributed to environmental insult or viral infections that can induce chronic liver damage and tumor development. One of the key genetic lesions found in liver cancer is inactivation of the retinoblastoma tumor suppressor (RB) pathway. This pathway is disrupted in most liver tumors;however, the role of RB loss in tumor etiology and progression remains largely undefined. Classically, RB is believed to inhibit cell cycle progression and serves as a critical deterrent to hyperplastic proliferation. To determine the mechanism(s) by which RB prevents tumor formation in the liver, we developed a mouse model of liver specific RB deletion. While RB loss causes transient alterations in cell cycle control, no hyperplasia was observed. Rather, RB-deficient hepatocytes undergo aberrant rounds of DNA replication leading to alterations in DNA ploidy suggesting a role for RB in the maintenance of genome integrity. The effects of RB loss are exacerbated by challenge with hepatocarcinogens, wherein the RB loss synergized with carcinogenic exposure to promote genome instability. These effects on genome stability are associated with a striking predisposition of RB-deficient livers to tumorigenesis. Combined, these data reveal novel mechanisms of RB function, and suggest that RB action in this tissue is distinct from its ability to prevent unchecked cellular proliferation. Rather, our data support the hypothesis that RB loss cooperates with hepatic carcinogens to induce genomic instability and promote liver tumorigenesis. We challenge this hypothesis and the role of RB in liver cancer by: determining the mechanisms through which RB loss compromises cell cycle control to alter genome stability in the liver (Aim 1);delineating the impact of RB loss on genome stability and hepatocarcinogenesis under specific pathologic conditions (Aim 2);and defining the action of temporal gene/environment and gene/gene interactions on liver tumorigenesis (Aim 3).